Detection of disease biomarkers (e.g., metabolites, proteins, cells) existing in body fluids (blood, urine, etc.) at low concentrations is usually conducted based on biological reactions such as enzymatic reactions and antigen-antibody interactions. Enzymes and antibodies recognize their targets with very high specificity and also exhibit high reaction efficiency, thus allowing detection of the analyses with high sensitivity. It is important to develop a diagnostic system based on the reaction characteristics to early defect disease-causing biomarkers and to allow for proper treatment in early stages. However, most of the currently available diagnostic systems are limited to laboratory since they require handling of reagents and devices and special techniques for diagnosis.
Recently, as a category in immunoassay, the necessity for early detection of biomarkers that allow estimation of symptoms and progress of human diseases, such as hormones, proteins and microorganisms, is increasing rapidly not only in medical institutions such as hospitals, emergency rooms, etc. but also at home.
For this, an immunoassay system that can be used conveniently in short time without requiring special knowledge or a complicated process is necessary. In general, diagnosis may be accomplished by an immunochromatographic method using a porous membrane as a support for immobilizing a sensing protein (e.g., an antigen or an antibody). If a sample containing an analyte is absorbed into a membrane strip from below, the analyte is transferred by capillary action to the immobilized sensing protein layer through pores. Antigen-antibody interaction occurs on solid surface and unbound components are separated by fluid flow. The membrane strip immunochromatography techniques based on this principle provides a convenient one-step diagnosis in which quick analyte detection is accomplished simply by adding a sample as the transfer of material is accelerated using the lateral flow of fluid.
The demand on the self-diagnostic system is reflected well in the recent rapid market growth of self-diagnostic kits for testing pregnancy and ovulation. In addition, as the establishment of Internet-based remote diagnosis and prescription is expected in the future, a home monitoring system will become a key element for diseases requiring regular diagnosis such as adult diseases.
However, most of the currently available self-diagnostic kits provide only qualitative analysis based on simple immunochromatographic analysis and visual inspection and they are not suitable for the analysis of disease biomarkers (e.g., proteins) that require high-sensitive quantitative analysis such as adult diseases. Although a colorimetric signal generated by a gold conjugate may be converted to optical density for quantitative analysis using an existing optical signal converting means, the detection sensitivity is low as compared to the enzyme immunoassay method widely used in laboratories.
The low sensitivity may be overcome by using high-sensitivity sensing materials such as fluorescent materials or radioisotopes. Indeed, an immunoassay system which performs membrane strip immunochromatographic analysis using a detection antibody labeled with a fluorescent material and interpreting the quantitative result using a fluorescence detector was developed. Recently, this technique has been applied to on-site immunoassay devices that can be used, for example, in emergency rooms because it provides high sensitivity and lacks hazardous elements. However, since the fluorescence detector is relatively too expensive and cannot be made into small size to be porthole, the system is used limitedly in hospitals or clinical laboratories and provides little advantage over the laboratory-scale enzyme immunoassay method except that the analysis time is shorter.
Furthermore, the enzyme immunoassay conducted in laboratories necessarily requires washing in each step of immune reactions to separate the immunoconjugate from unreacted materials and an enzymatic reaction has to be conducted separately for generation of signals. Accordingly, such a complicated multi-step process is not suitable for on-site diagnosis.
Therefore, an optical biosensor capable of easily analyzing a sample with high sensitivity without requiring expensive equipment or reagents is keenly needed.